1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit that uses the Thomson effect to achieve an enhanced heat discharge efficiency and an enhanced heating efficiency.
2. Discussion of the Related Art
Cathode ray tubes (CRT) have been generally used as the display systems for equipment such as televisions, monitors of metering devices, and data terminal equipment. However, such CRTs have a heavy and bulky structure that makes them unsuitable for appliances requiring a light and compact display.
Various alternative display devices have been developed to substitute for the CRT as displays when the size and weight limitations of CRT based displays do not provide the compactness and lightness desired for an appliance. Such display devices include the liquid crystal display (LCD) using an electro-optical effect, the plasma display panel (PDP) using gas discharge, and electro-luminescent (EL) displays using electro-luminescent phosphors. In particular, research on LCDs is being actively pursued.
Because LCDs have the advantages of compactness, lightness and low power consumption they have been actively developed as a substitute for CRTs. Recently, the development of LCDs has sufficiently advanced to enable LCDs to function effectively as flat panel display devices. As a result, LCDs are used not only for monitors of laptop computers, but also for monitors of desktop computers and other large-size information display devices, thus greatly increasing the demand for LCDs.
Most LCDs use the controlled transmission of light to generate a displayed image. Because the LCD does not itself emit light, a separate, external light source, namely, a backlight unit is required to illuminate the LCD.
Generally, backlight units are classified as either edge type or direct type in accordance with the arrangement of the cylindrical fluorescent lamp or lamps.
In the edge type backlight unit, a lamp unit is arranged at one side of a light guide plate. The lamp unit includes a lamp that emits light, lamp holders that are fitted around opposite ends of the lamp to hold the lamp, and a lamp-side reflection plate fitted into one side of the light guide plate such that the reflection plate surrounds an outer peripheral surface of the lamp and reflects the light emitted from the lamp toward the light guide plate.
The edge type backlight unit is primarily used with LCDs having a relatively small size, such as an LCD used as the monitor of a laptop computer or desktop computer. The edge type backlight unit exhibits excellent light uniformity, has a long lifespan and has an advantageous slim profile.
On the other hand the direct type backlight unit has actively been developed concurrently with the development of LCDs having a size of 20 inches or more. In the direct type backlight unit, a plurality of lamps are arranged in an aligned manner beneath a diffusion plate such that the lamps directly radiate light unto a front surface of an LCD panel.
The direct type backlight unit is mainly used in large-size LCD devices requiring high brightness because the direct type backlight unit exhibits a high light use efficiency compared to that of the edge type backlight unit.
However, where the direct type backlight unit is applied to a large-size LCD device such as the display of a large-size monitor or television, the operating time of the backlight unit and the number of lamps used in the backlight unit are increased compared to the operating time and number of lamps in used in the edge type backlight unit of the monitor of a laptop computer. The larger number and greater operating time of the lamps in the direct type backlight arrangement results in an increased possibility of lamp failure and in a reduced lifespan of the lamps.
For either the edge type or the direct type LCD backlight an EL lamp, a light emitting diode (LED), a cold cathode fluorescent lamp (CCFL), a hot cathode fluorescent lamp (HCFL), or an external electrode fluorescent lamp (EEFL) may be used.
Hereinafter, a backlight unit in accordance with the related art will be described with reference to FIGS. 1, 2, and 3.
As shown in FIG. 1, the related art edge type backlight unit includes a cover bottom 1, a lamp or lamps 2 arranged above the cover bottom 1 at one side or either side of the cover bottom 1, and a lamp housing 3 that encloses the lamp 2 except for a an open light emission face to allow light from the lamp 2 to exit the lamp housing 3.
Although not shown, the edge type backlight unit also includes a light guide plate that guides light emitted from the lamp 2, reflecting the emitted light toward an LCD panel; a diffusion sheet that diffuses the light emerging upwardly from the light guide plate through a certain angle; a prism sheet that condenses the diffused light and transmits the condensed light to the LCD panel; and a bottom-side reflection plate that upwardly reflects light transmitted to the cover bottom 1, in order to minimize loss of light. A plurality of diffusion sheets may be used as required.
Where the lamp 2 is arranged at either side of the cover bottom 1, the backlight unit may be used in a monitor. On the other hand, where the lamp 2 is arranged at one side of the cover bottom 1, the backlight unit may be used in a notebook computer. Although the lamp 2 is shown in FIG. 1 to have a straight structure, the lamp 2 may have an L-shaped or U-shaped structure.
In the edge type backlight unit of FIG. 1, the temperature of the lamp 1 and the temperature of a light incidence region to which light emitted from the lamp 1 is directed increase during continuous driving of the lamp 1.
FIGS. 2 and 3 show in detail the temperature profile of the lamp and the light incidence region designated by “A” and the region designated “B” spaced away from the lamp. The illumination from the lamp results in Region “A” having a higher temperature than region “B”.
Increased temperatures of the lamp 1 and of the light incidence region accompany the overheating of the lamp from continuous driving of the lamp and result in a degradation of the performance of the lamp a reduction in the lifespan of the lamp.